The present invention relates generally to wireless spread spectrum communications systems and in particular, embodiments to such systems, processes and devices which increase the data rate and performance of spread spectrum communications.
Modern communications electronics employ a variety of schemes for using radio spectrum to communicate. For example some systems employ frequency division multiple access (FDMA) schemes, in which the frequency band is divided into a series of frequency slots and different transmitters are allotted different frequency slots.
Other systems employ Time Division Multiple Access (TDMA). In time division multiple access the time that each transmitter may broadcast is limited to a time slot. In a TDMA system the frequency upon which each transmitter transmits may be constant or may be continuously changing (frequency hopping).
Yet other systems employ the use of code division multiple access (CDMA), or spread spectrum schemes. In a CDMA system, users transmit on the same frequency band. Each CDMA system has a dedicated code that is used to separate that system""s transmission from all others. These codes are commonly referred to as spreading codes, because the information density is small compared with the bandwidth size. These codes are also commonly referred to as a Pseudo Noise (PN) codes, because their seemingly random arrangement contains many of the characteristics of a pure noise signal. In a CDMA transmission, each bit of data to be transmitted is replaced by a spreading code if the data to be transmitted is a xe2x80x9c1xe2x80x9d, and is replaced by the inverse of the spreading code if the data to be transmitted is xe2x80x9c0xe2x80x9d.
To decode a spread spectrum transmission at the receiver, it is necessary to xe2x80x9cdespreadxe2x80x9d the code. One way to despread the code is to multiply the incoming signal, bit by bit, with the spreading code and sum the result. The process of multiplying the incoming signal bit by bit is commonly known as correlation and, upon multiplication, the signal is said to be correlated with the code. Despreading the incoming signal with a particular code involves separating the original data, encoded with that particular code, from all other CDMA transmissions, and to recover the original data. It is a property of PN codes that the presence of one PN code signal does not change the result of correlating the signal with another code. In other words the signal may be decoded using the proper PN code despite the fact that many other CDMA signals may be present. The property that one code does not interfere with the detection of the presence of another code is often referred to as orthogonality, and codes which possess this property are said to be orthogonal.
Spread spectrum has a number of benefits. One benefit of spread spectrum transmission is that, because the data transmitted is spread across the spectrum, spread spectrum can tolerate a great deal of interference. Another benefit of spread spectrum transmission is that messages can be transmitted with low power and still be decoded.
In addition to the different signal modulation techniques and frequency band utilization in wireless communication applications, various systems use different ways of timing communication between communicating units. Two basic schemes for timing communication between units are full and half duplex. When using a full duplex protocol, the communicating units can transmit and receive messages simultaneously. However, with a half duplex protocol, when one unit is transmitting the other unit is prohibited from transmitting.
In half duplex systems, the same frequency can be used for both transmission and reception, thereby enabling the transmit and receive circuitry to share many common components. In addition to the ability to share common components, half duplex schemes are further advantageous in that less isolation is required between the transmit and receive circuitry, because transmitting does not occur at the same time as receiving.
A variant of half duplex mode, which may be used in such applications as cordless phones, is Time Division Duplex (TDD). In such cordless phone systems using the TDD protocol the handset and the base unit may alternatively transmit during allotted time slices, so that the handset and the base unit do not transmit at the same time. Even though in cordless phone applications, a base station and a handset may use a Time Division Duplex protocol, TDD systems may be used to provide seemingly bi-directional communications. Speech can be compressed so that a spoken word will take less time to transmit then it does to say. Using audio compression and decompression techniques, the user perceives simultaneous bi-directional audio communications, and is unaware of the slight time delay necessary to compress and decompress the audio signal.
As popularity increases for wireless communications electronics, such as cordless and cellular telephones, wireless units to transmit music to speakers or headphones, programmable digital assistants and computers with wireless links, pagers, and wireless tracking devices, the demand for cheaper, higher performance devices also increases.
Manufacturers of wireless consumer goods have improved performance, for instance, by employing spread spectrum technology in communications applications. Spread spectrum technology can enable greater range and a significant increase in noise immunity for a given transmit power, when compared to many other wireless technologies. Spread spectrum technology also offers improved security, because the pseudo random nature of the codes used to produce the spread spectrum signal make the signal characteristics very similar to noise and, thus, the probability of accidental or deliberate reception by a third party is reduced.
Manufacturers of consumer goods have made efforts to reduce costs, for instance, by producing integrated circuits that can be used in a variety of applications. By producing integrated circuits that can be used in a variety of applications the number of integrated circuits of a given type, which are produced, can be increased. By increasing the number of integrated circuits of a given type, that are produced, the costs of design and production can be amortized over a larger number of units, and thus the cost per unit is reduced.
As a result, manufacturers of consumer goods have a need to minimize the number of different integrated circuits used in similar applications, and to manufacture integrated circuits that can be used in a variety of applications and yet provide good performance in all of them.
One of the difficulties of providing a circuit that can serve in many applications is that different applications can have different requirements. For example, in a cordless portable phone the data rate is, on average, equal in both directions. In portable phones, increased broadcast range is a desirable feature. Both the range of the phone and the bi-directional data rate may be important considerations. In wireless speaker applications, several channels of high fidelity sound must be carried a short distance, but only in one direction. In personal digital assistants, the desired data transmission rate can fluctuate. For example a high data rate is needed on the receive end when receiving e-mail and a high transmission rate is needed on the sending end when transmitting e-mail. In a portable computer there may be a need for a very low data rate and noise immune transmission capability, to transmit commands to an Internet service provider, and a need for a high data rate, to download web pages from the service provider. The receive data rate, transmit data rate, range, noise immunity, sensitivity, and security are all factors that influence each other in wireless applications and may change from application to application. In addition, even in a single application, the requirements may vary, depending on, for example, operating conditions or environment of use. For example, the clarity and fidelity of sound in a portable phone may be an important consideration but, as the signal starts to fade, a user may willingly accept a somewhat lower fidelity in order to have an increased range.
Many of the common wireless integrated circuits provide for a receive data rate, transmit data rate, range, noise immunity, and sensitivity, and security that are tailored to a particular application or operating condition. There is a need for integrated circuits with greater flexibility with different applications and adapt to changing conditions within applications.
Although the term CDMA is used widely to describe a type of telephone communication, the term spread spectrum may also be applied, and is used interchangeably herein. Spread spectrum, or CDMA is also applicable to many wireless communications systems and is not limited to use within portable phones.
The process of extracting data from a CDMA signal is commonly known by many terms such as correlating, decoding, and despreading. Throughout this disclosure these terms will be used interchangeably.
The codes used by a spread spectrum system are commonly referred to by a variety of terms including, but not limited to, PN (Pseudo Noise) codes, PR (Pseudo Random) codes, spreading codes, despreading codes, and orthogonal codes Throughout this disclosure these terms will be used interchangeably.
Accordingly, preferred embodiments of the present invention are directed to systems which incorporate wireless spread spectrum technology. Cordless and portable telephones, and wireless data communication are examples of systems which may employ spread spectrum technology, and preferred embodiments within these systems are used to facilitate the disclosure.
Amplifiers within wireless communication systems typically have preset bandwidths. The amplifier bandwidth is chosen to amplify desired signals and to attenuate signals that fall outside of that band. Amplifiers, in general, are designed for bandwidth considerations. An amplifier designed to amplify a data signal with a data rate of, for example, 10 Megahertz may be able to amplify signal having a data rate of 10 Megahertz representing raw data of 10 Megahertz, or a signal having a 100 Kilohertz data rate multiplied by a 100 bit spreading code. In other words, once the data rate to the amplifier is set, the composition of the signal, whether it is pure data or data that has been multiplied by a spreading code, may be of little consequence to the amplifier.
Amplifiers within wireless spread spectrum systems have considerably wider bandwidth than amplifiers in similar applications which do not employ spread spectrum technology. This is because the spreading code multiplies the data rate of the communications signal by the length of the spreading code. This higher spread spectrum data rate, which is input to the amplifier, commonly called chipping rate, must be accommodated by amplifiers within the wireless system. Thus, such amplifiers must be able to accommodate the higher chipping rate.
There may be occasions where the spreading code size can be reduced or eliminated. If the spreading code can be reduced in length, or eliminated, then the relatively large bandwidth of such an amplifier, to support the spread spectrum coding, may instead be used for increased data rate transmission. Conversely, if the data rate is reduced, longer PN codes can be used to thereby increase the noise immunity and the transmission range of the signal.
In addition many wireless systems employ a half duplex communications scheme called TDD. TDD allows seemingly concurrent communications in both directions (transmit and receive) in such applications as portable phones, by rapidly switching between receiving and transmitting modes. In many TDD systems the transmit and receive portion of the TDD cycle each occupy approximately 50% of the communication time. A duty cycle of 50% may be chosen for convenience. In such applications as cordless phones a 50% transmit/receive duty cycle may be able to easily support the maximum data rate in each direction. However by increasing the transmit portion of the duty cycle, more data can be transmitted. Conversely by increasing the duty cycle of the receive portion of the duty cycle, more data can be received.
These techniques, alone and in combination, can be used to increase the flexibility of circuits within wireless communication systems. Circuitry that can accommodate these techniques can be used in a wider variety of wireless systems than those which must be individually tailored to the particular wireless system in which they are embedded. These techniques alone and in combination can also add flexibility within a single application, for example increasing data throughput or range as application conditions demand.